The 87 million Facebook users had their data harvested by a third-party app posing as a personality quiz. Advertisers choose the types of users they want to reach. He started off with an apology to United States lawmakers on Tuesday as he made a long-awaited appearance in the Congressional hearing since the data breach scandal broke. "Why should we trust you?" asked Democratic Representative Mike Doyle. The firm that has been doing its own damage control to respond to questions that it used wrongly or illegally acquired data to manipulate up to 71 million Americans (over 87 million users' data was sold in total). It also reignited long simmering concerns about Facebook's impact on the world's privacy, civil discourse and domestic institutions.

He declined to commit to changing all accounts' default settings to collect the minimum amount of data, saying it was a "complex issue" which deserved more than a yes or no answer.

"Do not consider ourselves as a financial institution though we help people send each other money". In two days of testimony, Zuckerberg was asked by at least four different lawmakers about the conservative duo and allegations Facebook censors conservative voices. "We have to make sure people aren't using it to harm people or spread disinformation", Zuckerberg told the Senators.

Zuckerberg disclosed that his company is "working with" special counsel Robert Mueller in the federal probe of Russian election interference.

As it turns out, CEOMark Zuckerberg himself may have gotten one of those automated warnings, because his personal data was included in CA's roundup, too. Going forward, developers will also have to sign a contract in order to access a user's posts or private data.

Talking about user data and the control users have on their own data, Zuckerberg explained that there's a clear approach that the company takes where users have to opt-in to access an app on the platform. But Facebook failed to conduct an audit to ensure that Cambridge Analytica had actually deleted the data.

Mr Zuckerberg told the committee that Facebook found a "whole programme associated with Cambridge University" where a number of researchers built similar apps to that developed by Alexander Kogan.

The senators asked Zuckerberg how he'd feel if his personal information was compromised.